The present invention relates generally to medical devices and particularly to electropolishing medical implants.
Electropolishing is a widely used manufacturing process that provides a smooth surface finish to metallic parts. Typically, electropolishing is used after various forming operations, such as machining, punching, laser cutting, and electrodischarge cutting, to remove burrs, sharp edges and other rough features that are generated during the manufacture of metallic parts.
The basic concepts of electropolishing are well known to those in the art, and thus, only a brief summary is required here. Conventional electropolishing processes involve contacting a metallic part with an anode (i.e., a positively charged electrode) and spacing a cathode (i.e., a negatively charged electrode) away from the metallic part. The metallic part, along with the anode and cathode, are then immersed in a bath of electrolytic fluid. Next, a voltage is applied across the anode and the cathode for a period of time. The effect of this is that metal from the metallic part is drawn away from the metallic part and is drawn to the cathode. (Although different in some respects, electropolishing may be thought of conceptually as the opposite of electroplating.) Because burrs and sharp edges experience a higher current density than smoother surfaces on the part, metal is removed from these areas at a faster rate than the rest of the metallic part. Thus, electropolishing processes leave a smooth surface finish in which the rough edges of the metallic parts are removed.
One application in which electropolishing is particularly useful is for finishing endovascular stents and other medical implants. Medical implants require exceptionally smooth surfaces since any rough edges may cause tissue irritation during or after being implanted into a person's body. Some of the medical problems that may be encountered when rough edges are not properly removed from a medical implant include inflammation, bleeding and/or scarring of the surrounding tissues. In the case of endovascular stents, such conditions can be particularly harmful and dangerous. For example, one risk that may result from the use of stents with rough edges is restenosis. Restenosis refers to the re-narrowing of a vessel which sometimes occurs after balloon angioplasty procedures. Although restenosis may occur for a number of reasons, tissue irritation and disturbance caused by rough edges on a stent may be one cause of restenosis.
Various apparatuses for electropolishing stents have been tried.
One such apparatus involves wrapping a platinum wire (i.e., the anode) around the outer surface of the stent. The stent is then lowered into an electrolytic both in a horizontal orientation (i.e., with the two ends of the stent being positioned at approximately the same height above the bottom of the bath). The cathode is formed as a single horizontal loop that surrounds the stent (i.e., the loop defines a plane that is approximately parallel to the bottom of the bath).
This apparatus suffers from several problems, however. One problem is that marks are generated on the surface of the stent around the points of electrical contact between the platinum wire and the stent. This is a common problem with electropolishing apparatuses and is not limited to the particular electropolishing apparatus described here. This problem occurs because the area of the stent located near the electrical contact between the wire and the stent experiences a higher current density than the rest of the stent. As a result, metal is drawn away from this area of the stent at a particularly aggressive rate. In addition, the wire effectively masks the portion of the stent which is in direct contact with the stent, thus creating an area that experiences a minimal rate of metal removal. The result of this arrangement is that small grooves, pits and other marks are formed around the electrical contact in a random pattern. Thus, the smooth surface finish which is desired across the entire stent is not achieved due to the marking that occurs around the electrical contact.
Another problem with this apparatus is that the metal removal rate is not uniform across the entire stent. One problem is that the ends of the stent generally experience a higher metal removal rate than the center. This is caused in part by the closer proximity of the ends of the stent to the cathode. In contrast, the center region of the stent is located at or near the center of the cathode loop (i.e., farther away from the cathode loop itself). In addition, since the anode (i.e., the platinum wire) is wrapped around the outer surface of the stent, the inner surface of the stent experiences a lower metal removal rate than the outside surface of the stent. In addition, because the anode (i.e., the platinum wire) is wrapped around the outer surface of the stent, the inner surface of the stent may experience a lower metal removal rate than the outside surface of the stent.
Uneven metal removal is a problem that many electropolishing apparatuses suffer from. In the case of stents, this problem can make manufacturing more difficult and expensive since manufacturing tolerances need to be especially tight in order to ensure proper performance of the stent. Thus, in electropolishing processes in which the metal removal rate varies significantly across the stent, the percentage of manufacturing rejects may be higher, thereby raising costs.
Other typical electropolishing apparatuses include tree-like racks having a vertical center-stem and angled arms extending out from the center-stem. Stents are installed on each of the arms by sliding the stent over an arm so that the arm extends through the cylindrical cavity of the stent. Therefore, the tree-like rack functions as the anode by contacting the inner surface of the stent. The cathode may be a cathode like that previously described or may be a metal container that holds the electrolytic fluid.
This apparatus, however, suffers from problems that are similar to those already described. For example, marking around the electrical contact between the anode and the stent may also be a problem with this apparatus. In addition, the diameter of the arm that extends through the center of the stent typically fills most of the center cavity of the stent. The reason for this is that the arms usually need to be built strong to avoid deforming the arms during loading, unloading and normal manufacturing use. The problem with this design is that the large diameter of the arms prevents electrolytic fluid from circulating within the interior of the stent. As a result, the interior surfaces of the stent do not receive a consistent polish.
It is apparent to the inventor that an apparatus and method for electropolishing medical implants is desired in which marking of the medical implant is minimized and metal removal is more consistent. Accordingly, a solution is described more fully below which solves these and other problems.